A hybrid filter is a 3-port electronic device that has one sum port, one low-pass port and one high-pass port. Energies entering the sum port are directed to the other two ports based on the frequency characteristics of the signals being transported. Low-frequency signals are accordingly transported to the low-pass port, and high-frequency signals to the high-pass port. With these electrical properties, constant-resistance filters may be utilized in filtering applications and applications in which amplification of bi-directional signals travelling over a single transmission path is accomplished (i.e., repeaters).
In U.S. Pat. No. 2,115,138, S. Darlington described a wave transmission network, having a particular frequency selective property, which could be employed in a bi-directional repeater network to amplify signals—travelling in opposite directions along a single transmission path using—using only one amplifier. In U.S. Pat. No. 3,593,209, N. Gittinger described a hybrid filter having a frequency selective property, similar to Darlington's, which could be employed in a two-amplifier repeater node. Both of these patents employed two identical pairs of low-pass and high-pass T-networks in a bridge configuration, in conjunction with a phase inverting transformer, to form a collective wave transmission network or hybrid filter.